Does the "plasma" in plasma televisions need to be recharged or replaced over time?

No. This question is another one of what I like to refer to as an Electronics Industry Urban Myth! The plasma gases are sealed in individual cells when they are manufactured. There is no way to recharge or replace the gases. However, plasma TVs, when used normally, will last you many, many years (up to 20 years) so you don't have to worry about the screen burning out for a long time.